There has been considerable recent interest in the development of magnetic nanoparticle (mNP) technologies for diagnostic and imaging applications. Compared to larger magnetic particles, the smaller nanoparticles (NPs) display potential advantages in their diffusive and superparamagnetic properties. Magnetophoretic mobility, μm, is defined as the acceleration of an object in the presence of a magnetic field, which determines the ability to control the object's movement within a magnetic field. The μm for an individual particle at room temperature and above is defined as
      μ    m    =            π      ⁢                          ⁢              μ        0            ⁢              M                  S          ,          C                2            ⁢              D        C        5                    324      ⁢                          ⁢              k        B            ⁢      T      ⁢                          ⁢      η      
where μ0 is the magnetic constant, MS,C is the saturation magnetic moment of the mNPs, DC is the diameter of the mNPs, kB is the Boltzmann constant, η is the viscosity of the medium, and T is the temperature. Because of their small particle size, which results in randomized magnetic moments, the μm for mNPs is usually small. This leads to an intrinsic challenge for applications where the favorable diffusive properties of the small mNPs are advantageous, for example, where the mNPs are used to capture diagnostic targets via antibody-antigen interactions. On the one hand, the small particles display better association and binding properties, but on the other hand their small size reduces magnetic capture efficiency.
Approaches to overcoming the small μm for mNPs include using larger macromolecules or objects that are labeled with multiple mNPs, making the mNPs from materials with larger MS,C using irreversibly aggregated mNPs, or using high magnetic gradients. However, most of these approaches result in the loss of favorable diffusive properties and suffer some drawbacks in the microfluidic-based diagnostic device environment. High magnetic gradients require a large number of coils and high current, which cannot be easily integrated into microfluidic devices. Materials with high MS,C are usually metals or alloys and, because of their high surface/volume ratio, they are prone to oxidation events that can lower their MS,C. The pre-aggregation of mNPs into larger structures results in the permanent lowering of surface/volume ratio and to a decrease in the mNPs favorable diffusive properties. There is a need for mNPs with favorable diffusive properties that can also be readily separated in a small magnetic field.
Stimuli-responsive (“intelligent” or “smart”) materials and molecules exhibit abrupt property changes in response to small changes in external stimuli such as pH; temperature; UV-visible light; ionic strength; the concentration of certain chemicals, such as polyvalent ions, polyions of either charge, or enzyme substrates, such as glucose; as well as upon photo-irradiation or exposure to an electric field. Normally these changes are fully reversible once the stimulus has been removed.
Poly(N-isopropylacrylamide) (PNIPAAm) is a temperature-responsive polymer that exhibits a lower critical solution temperature (LCST) around which the polymer reversibly aggregates. Below the LCST, PNIPAAm chains hydrate to form an expanded structure; above the LCST, PNIPAAm chains dehydrate to form a shrinkage structure. This property is due to the thermally-reversible interaction of water molecules with the hydrophobic groups, especially the isopropyl groups, leading to low entropy, hydrophobically-bound water molecules below the LCST and release of those water molecules at and above the LCST. Modification of mNPs with PNIPAAm yields particles that can be reversibly aggregated in solution as the temperature is cycled through the LCST.
Previous work with PNIPAAm-modified mNPs has relied on post-synthesis chemical modification of the particles. Chiu et al. synthesized a Fe3O4 ferrofluid by co-precipitating FeCl3 and FeCl2. The ferrofluid was then mixed with a PNIPAAm solution and crosslinked to form magnetic polymeric networks. Lin, C. L. and W. Y. Chiu, J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 5923-5934. Wang et al. also co-precipitated FeCl3 and FeCl2 to synthesize Fe3O4 particles. Deng, Y., et al., Adv. Mater. 2003, 15, 1729-1732. The particles were coated with a layer of silica and modified with 3-aminopropyltrimethoxysilane to seed the precipitation polymerization of NIPAAm. In both methods, the post-synthesis functionalization requires multiple steps and can result in particle aggregation. There is a need for methods of making stimuli-responsive polymer-modified mNPs that do not require extensive post-synthesis workup steps and result in minimal particle aggregation.
Stimuli-responsive materials and molecules have numerous possible applications in the biomedical/pharmaceutical field, as well as in biotechnology and the related industries. Smart conjugates, smart surfaces, smart polymeric micelles, and smart hydrogels have all been studied for a variety of diagnostics, separations, cell culture, drug delivery, and bioprocess applications.
Despite the development of magnetic nanoparticle (mNP) technologies for diagnostic and imaging applications, there exists a need for a stimuli-responsive magnetic nanoparticle with favorable diffusive properties as well as with the ability to be reversibly aggregated into larger structures, and simpler methods for making the nanoparticles. The present invention seeks to fulfill these needs and provides further related advantages.